Heretofore, hollow structural bodies have been produced by various methods. For example, Patent Document 1 discloses a hollow ceramic fiber product and a method for producing the same. The Patent Document describes that the hollow ceramic fiber product is made of a metal oxide having a thickness of 0.1 μm or more precipitated on a peripheral surface of organic fibers from a solution containing a metal compound serving as a precursor, and that holes corresponding to the shape of the organic fibers are formed inside by removing the organic fibers.
However, the hollow ceramic fiber product is made of an inorganic compound of ceramics and has poor lightweight feel compared with a fiber product made of an organic compound. Furthermore, since a metal oxide film is formed by immersing an organic fiber in a solution containing a metal compound serving as a precursor, it is difficult to control a thickness of a coating film.
Also, there are high needs for a fiber having functionalities such as lightweight feel and heat retaining properties. Therefore, it is widely used to produce various hollow synthetic fibers made of acryl, polyester, nylon, and the like.
Examples of the method for producing a hollow synthetic fiber include a method in which a hollow structural body is formed during a spinning process using a spinneret, and a method in which a woven knit product is produced using fibers composed of two components and the fiber of any one of components is dissolved to form a hollow structural body. The former method is disclosed in Patent Documents 2 to 6. The latter method is disclosed in Patent Document 7.
However, in the former method, regardless of wet spinning, dry spinning or melt spinning, a large-scale device is required. Since the production method varied with a difference in a fiber material, production cost is high and skilled technique and knowledge are required, and also chemical resistance is poor. Furthermore, the above production method is a technique which can be applied only for fibrous products and it is difficult to apply for various shapes and sizes, like a film. Since it is also a technique of forming a hollow structure during the production process, it is difficult to form a hollow structure in commercially available products.    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 2001-248024    Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No. 9-78355    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 2003-105627    Patent Document 4: Japanese Unexamined Patent Publication (Kokai) No. 2005-256243    Patent Document Japanese Unexamined Patent Publication (Kokai) No. 2006-45720    Patent Document 6: Japanese Unexamined Patent Publication (Kokai) No. 2006-9178    Patent Document 7: Japanese Unexamined Patent Publication (Kokai) No. 2007-016356